A logistical problem for gene therapy remains low levels of gene transduction in long-term marrow engrafting hematopoietic stem cells (HSC). Recent advances from our laboratory suggest we are now in position to address enhancing gene transduction in HSC and progenitor (HPC) cells. We reported that IL-20 is unique in enhancing the proliferation of murine and human CFU-GEMM without effects on other HPC, Stromal Cell Derived Factor-1 (SDF-1/CXCL12) enhances survival of and antiapoptotic activity for murine HSC and HPC and human HPC, and enhances Moloney-type retroviral gene transduction of murine and human HPC, and that the bicyclam AMD3100, an antagonist of SDF-1/CXCL12 binding to CXCR4 acts as a potent mobilizer ol HSC and HPC in mice, and HPC in humans and enhances mobilization induced by G-CSF, even in poor responders to G-CSF. We also demonstrated a role for CD26 (Dipeptidylpeptidase IV) in homing/engraftment of HPC/HSC. Towards our long-term goals to enhance efficiency of gene transduction for clinical therapy, we propose three Specific Aims: 1) Elucidate the SDF-1/CXC12-CXCR4 axis and inhibition or deletion of CD26 along with Retronectin, and the combination of Stem Cell Factor, FIt3-Ligand, and Thrombopoietin, with and without IL-20, for their capacity to enhance efficiency of Moloney-type retroviral, and lentiviral vector gene transfer into long term engrafting HSC, and into HPC from marrow. Assays include competitive repopulation of lethally irradiated mice with mouse HSC and repopulation of sublethally irradiated NOD/SCID mice with human HSC and also transplantation into secondary mouse recipients; HPC: in vitro colony assays; 2) Evaluate gene transduction efficiencies in: i) HSC and HPC mobilized to peripheral blood by AMD3100 plus G-CSF, compared to G-CSF, and ii) cord blood, each in comparison to marrow; 3)Determine effects of inhibition of CD26 on the long-term engrafting capability of gene-transduced mouse bone marrow and AMD3100 plus G-CSF-mobilized mouse peripheral blood. These studies should enhance our knowledqe of .qene transduction into Ion.a-term marrow enqraftinq HSC.